Stereoisomerism. 1 67 



For a solution 



lOO 



where p is the number of grammes of active substance in 

 100 c.c. of solution, and d is the density of the solution. 



Pasteur's Researches on the Tartaric Acids. The dis- 

 covery of the isomeric tartaric acid (racemic acid) by 

 Berzelius in the mother liquors of the tartaric acid has 

 been already mentioned. The two known tartaric acids 

 were subsequently investigated by Mitscherlich, who found 

 generally a marked similarity between the sodium am- 

 monium salts of the two acids, but showed that whereas 

 the solution of the ordinary tartrate rotated the plane of 

 polarized light to the right, the corresponding salt of the 

 racemic acid was optically indifferent. The investigation 

 of this difference in properties between the two acids 

 forms the starting point of the classical investigations of 

 Pasteur. It is of interest to read the account of Pasteur's 

 endeavours to obtain the racemic acid, as related in his 

 biography by his son-in-law, Vallery-R-adot ; for it appears 

 that after the first discovery of this acid by Berzelius, its 

 appearance in the mother liquors of the tartaric acid 

 manufacture was rare, and it was only after a somewhat 

 exciting search through factories that sufficient material 

 for investigation was obtained. 



In the pursuit of his studies on the two isomerides 

 Pasteur was guided by certain observations of his prede- 

 cessors. The discovery of polarized light had been made 

 in 1808 by Malus, and his investigations were continued 

 after his early death by his pupils Arago and Biot. The 

 latter had found that there are two kinds of quartz crystals, 

 one of which rotates polarized light in one direction, 

 whereas the other rotates it in the opposite direction. 

 Now, two kinds of quartz crystals had been described by 



